A ribbon microphone includes a microphone case accommodating a ribbon microphone unit, a step-up transformer, a circuit board, a connector, and any other component. The ribbon microphone unit includes, as its main components, two magnets generating a magnetic field and a conductive ribbon. These magnets are arranged on the two sides of the ribbon, and a magnetic field is generated between these magnets. The ribbon is disposed in the magnetic field while two ends in its longitudinal direction are held under proper tension. The ribbon vibrates in the magnetic field in response to sound waves, and a current corresponding to the vibration flows through the ribbon. In this manner, the sound waves are converted into electric signals. Each magnet has a rod shape which has a rectangular cross-section. The two magnets are arranged in parallel with each other while one surface in the width direction of one of the magnets faces that of the other magnet across the ribbon. An aluminum foil has been widely used as the material for the ribbon. Aluminum has higher conductivity and a lower specific gravity than any other metallic material and is thus suitable for a ribbon of a ribbon microphone.
A typical conventional ribbon microphone unit is configured such that one ribbon is arranged in one magnetic field generated by magnets. Another commercially available ribbon microphone has two ribbons that are arranged at a predetermined space in parallel with each other in one magnetic field and that are connected in series. With this configuration, the ribbon microphone can produce an output of double magnitude. Such a double-ribbon microphone unit is disclosed in Japanese Patent Laid-Open No. 2009-118118 issued to the assignee of this application.
In a ribbon microphone unit including two ribbons as disclosed in Japanese Patent Laid-Open No. 2009-118118, ribbons are arranged at two ends in the anteroposterior direction of magnetic poles, i.e., at positions corresponding to two ends in the thickness direction of magnets. The two ribbons are electrically series-connected as described above. Since aural signals outputted by the two ribbons are weak, the signals are outputted as a microphone output after the voltage of the signals is raised with a step-up transformer. A ribbon microphone is bidirectional, and the front and rear ribbons are set equally in acoustic terms such that aural signals produced by the front and rear ribbons are bidirectional.
FIG. 3 shows a conventional ribbon microphone unit provided with two ribbons arranged in one magnetic field. The ribbon microphone unit 10 (hereinafter simply referred to as “unit 10”) includes a yoke 12, two magnets 15, and two ribbons 16 and 17. With reference to FIG. 2, which illustrates an embodiment of the present invention, the yoke 12 is a vertically long rectangular frame. The yoke 12 has the rod-shaped magnets 15 having a rectangular cross-section and fixed to opposed vertical inner walls, respectively, of the yoke 12 in parallel at a distance. These magnets 15 are magnetized in a direction orthogonal to the opposed surfaces of the magnets 15, i.e., a direction orthogonal to the sheet surface in FIG. 3, and the magnetic poles of the magnets 15 are oriented in the same direction. As a result, a parallel magnetic field with a magnetic flux oriented in one direction is generated between the magnets 15.
In the magnetic field, the two ribbons 16 and 17 are arranged. The two ends in the longitudinal direction of each ribbon 16 or 17 are fixed under proper tension to respective terminal portions provided at the two ends in the longitudinal direction of the yoke 12. The ends of the ribbon 16 are electrically continuous with terminals 21 and 22 whereas the ends of the ribbon 17 are electrically continuous with terminals 23 and 24. One end in the longitudinal direction of each of the ribbons 16 and 17, i.e., the upper end in FIG. 3 is connected by a wire to the corresponding terminal portion of the yoke 12 via the terminal 21 or 23. The other end of the ribbon 16 is connected to one end of a primary winding 301 of a step-up transformer 30 via the terminal 22 whereas the other end of the ribbon 17 is connected to the other end of the primary winding 301 via the terminal 24. Accordingly, the ribbons 16 and 17 are connected in series such that output signals from the ribbons 16 and 17 are inputted to the primary winding 301 of the step-up transformer 30. The magnetic field extends over substantially the same range as the thickness of the magnets 15 (the lateral direction in FIG. 3 (the anteroposterior direction)), and the ribbons 16 and 17 are arranged near the two ends, respectively, in the anteroposterior direction of the magnetic field. This is because the unit 10 is not bidirectional unless the front and rear ribbons 16 and 17 are set equally in acoustic terms.
As shown in FIG. 3, sound waves v1 entering the ribbon microphone unit 10 from the front face of the ribbon 16 act on the ribbon 17. For convenience, sound waves acting on the ribbon 17 will be denoted by reference characters v2 hereinafter. The two ribbons 16 and 17 vibrate in response to the sound waves v1 and v2. Electromagnetic conversion causes currents i1 and i2 corresponding to the sound waves v1 and v2 to flow through the ribbons 16 and 17, respectively. Since the upper ends of the two ribbons 16 and 17 are connected in series via the terminals 21 and 23 in FIG. 3, the currents i1 and i2 flowing through the ribbons 16 and 17 are opposite in direction and are equal in magnitude. The current i1 (=i2) flows into the primary winding 301 of the step-up transformer 30.
The step-up transformer 30 is an output transformer of the ribbon microphone unit 10, has a turns ratio of as high as, for example, 1:70, and raises an output voltage of the unit 10 about 70 times and output the raised voltage. Not only a microphone unit including two ribbons as shown in FIG. 3 but also a microphone unit including one ribbon outputs an extremely low voltage. Accordingly, the step-up transformer has a turns ratio of as high as 1:70.